This invention relates to the field of thin film deposition techniques. More particularly the invention relates to sputtering techniques for thin films on magnetic disks and even more particularly to protective overcoats for disks used in data storage devices.
Sputtering is the physical ejection of material from a target as a result of ion bombardment of a target mounted on a cathode. The ions are usually created by collisions between sputtering gas atoms, e.g. argon, and electrons in a glow discharge. The ions are accelerated into the target attached to the cathode by the electric field which may be DC, AC or RF. A substrate placed in a suitable location will intercept some of the atoms which have been sputtered off of the target and a thin film coating of target material can be deposited on the substrate. The targets are eroded by this process and must be periodically replaced. In some applications a negative bias voltage is applied to the substrate. This method, termed bias sputtering causes positive sputtering gas ion (e.g. argon ion) bombardment of the coating during its growth, which can have beneficial effects in some applications. Some underlayers and magnetic layers are deposited using bias. The bias used in disk industry sputtering is normally DC. In reactive sputtering, a reactant gas, e.g. a hydrocarbon gas, ammonia, etc., is added to the chamber in addition to the sputtering gas to form one or more compounds with the target material in the deposited film.
In a planar magnetron sputtering system , the cathode assembly includes an array of permanent magnets arranged to form a closed loop magnetic field, commonly referred to as a xe2x80x9crace trackxe2x80x9d, which establishes the path or region along which sputtering or erosion of the target material takes place. In a magnetron cathode, the magnetic field confines the glow discharge plasma and increases the path length of the electrons moving under the influence of the electric field. This results in an increase in the gas atom-electron collision probability. This leads to a much higher sputtering rate than that obtained without the use of magnetic confinement. Further, the sputtering process can be accomplished at a much lower gas pressure. Magnetron sputtering can also be used in a so-called dual cathode arrangement where each of the two electrodes can serve as the cathode.
The thin film magnetic recording disk in a conventional drive assembly typically consists of a substrate with several layers of sputtered thin films deposited on both planar sides of the disk. There is typically at least an underlayer consisting a thin film of chromium (Cr), a Cr alloy, or NiP; a cobalt-based ferromagnetic alloy deposited on the underlayer; and a protective overcoat over the magnetic layer. One variation of the layer structure described above uses a very thin initial seed layer on the substrate to establish an appropriate nucleation base for the underlayer.
The protective overcoat serves to inhibit corrosion and reduce stiction between the sliders (or heads) and the disk surface, as well as, to protect the magnetic film from mechanical damage. Numerous materials have been used/proposed for overcoats, but currently carbon based films are commonly found in commercial magnetic disks. Relatively pure amorphous carbon films have been used, but carbon films with significant amounts of hydrogen and/or nitrogen have also been used. The C:Hx films are reactive sputter deposited using graphite targets while introducing hydrogen or hydrocarbon gases into the chamber. DC or RF magnetron sputtering or DC or RF diode sputtering of these films is known. (See, for example, Yamashita 5,045,165). In the context of sputtering a carbon protective film on a magnetic tape, U.S. Pat. No. 4,869,797 describes the use of a bias voltage of xe2x88x9210 V to xe2x88x92100 V being applied xe2x80x9cin the vicinity of the support and magnetic layer.xe2x80x9d
When nitrogen is added gas in the sputtering chamber with graphite targets C:Nx films are produced. In U.S. Pat. No. 4,664,976 Kimura, et al. teach the use of a carbon nitride protective layer deposited by RF sputtering using a carbon target in the presence of nitrogen gas. Kimura, et al. state that the ratio of carbon to nitrogen in the film should be from 4:1 to 3:2. In U.S. Pat. No. 5,567,512 Ga-Lane Chen, et al., describe a carbon based overcoat with a surface density of nitrogen atoms of 3-8xc3x971016 atoms/cm2 and a preferred thickness of 80-150 Angstroms. The film is deposited in an atmosphere of argon with 20-40 at. % nitrogen.
Haines, et al., in U.S. Pat. No. 5,232,570 describe a method of sputtering CxNy films with 10 to 40 at. % N. The method uses DC magnetron sputtering with 300-1200 V DC on the cathode and ground (or powered anodes) to reduce the electron bombardment of the disk surface.
Yamashita, at al. in U.S. Pat. No. 5,507,930 describe a method for sputtering a C:Hx film to increase the life of the graphite targets by imposing an AC voltage on the DC power supply to the target. Yamashita states that the sputter rate of carbon by RF sputtering is many times lower than DC magnetron sputtering and that RF sputtering has the additional drawback among others of requiring a complex impedance matching network. Yamashita also states that the use of what he calls low frequency RF sputtering in the range of several hundred kilohertz simplifies the matching network, but still has too low of a sputtering rate to be practical.
In U.S. Pat. No. 5,443,888 Murai, et al. describe a method of making a magnetic tape including forming a diamond-like carbon film on a polymer film by a plasma CVD using an 80 to 200 kHz power signal with a peak voltage of more 500 V and a hydrocarbon having at least 3 carbon atoms. Murai states that it is preferable to add a DC voltage of 1000 to 2500 V to the AC voltage.
In U.S. Pat. No. 5,714,044 Bibari, et al., describe a method of making a dual layer overcoat with the upper layer being carbon nitride film for use on magnetic disks. The method includes making the magnetic fields on confronting sides of the substrate have opposite polarities. The patent states that the nitrogen should be 5 to 50 percent of the gas atoms in the chamber.
E. Cutiongco, et al. have described experiments on deposition techniques for carbon nitride overcoat for magnetic disks. (xe2x80x9cTribological Behavior of Amorphous Carbon Nitride Overcoats for Magnetic Thin-Film Rigid Disks,xe2x80x9d J. of Tribology, July 1996, volume 118, p. 543). They used planar single cathode or dual-cathode systems both of which were configured for dc unbalanced magnetron sputtering. For some experiments the bias was xe2x88x92200 or xe2x88x92250 volts with 10 microsecond pulses at a 20 kHz rate. The other sputtering conditions were 0.5 and 1.0 kW target power, 4 mTorr total gas pressure with nitrogen partial pressure of 0.2 mTorr.
In an article entitled xe2x80x9cReactive alternating current magnetron sputtering of dielectric layersxe2x80x9d M. Scherer, et al. state that dielectric layers like Al2O3, SiO2 and Si3Ni4 have been produced using a reactive ac magnetron sputtering process using a two cathode arrangement. The frequency range was 10 to 100 kHz. The article indicates that a frequency above 10 kHz has the advantage of eliminating the well known problem of arcing. In the experimental details, each cathode is connected to one of two floating outputs of a mid-frequency 10 kW power supply operating at 40 kHz. Although the article refers to the system as having two xe2x80x9ccathodes,xe2x80x9d it is clearly pointed out that each of these electrodes functions as a cathode only during one half of the cycle and functions as anode during the other half.
Improved apparatus and methods for depositing thin films are described. The invention can be used to apply any films which benefit from the application of a negative bias voltage to the substrate which could include underlayers, magnetic layers and protective overcoats for magnetic disks as well as many other thin films. The invention enhances the effects normally achieved with negative bias voltage by connecting a midfrequency power supply across two electrodes in the sputtering system while pulsing a negative bias voltage on the substrate (work piece) where the film is being deposited. The invention increases bias currents relative to DC and RF sputtering which implies an increase in the density of the substrate biasing ions at the substrate. A detailed description will be given of the use of the apparatus and method to deposit an improved carbon based protective overcoat which includes hydrogen (C:Hx) and/or nitrogen (C:Nx) which can be used on a thin film magnetic recording medium. The overcoat is preferably sputtered using a graphite target with nitrogen and/or hydrogen gas being supplied into the sputtering chamber in addition to the sputtering gas.
The pulsing frequency is preferably from 1 kHz to 200 kHz. The midfrequency signal imposed across the two electrodes is preferably from 10 to 400 kHz. The method can be beneficially used with any other films and/or materials for other applications which can be improved by increased density of the substrate biasing ions at the substrate surface.
In one embodiment of the invention in a sputtering system which simultaneously deposits a film on two planar surfaces of a disk, at least one pair of dual electrodes is used on each side of the system. The phases of the power signals to each side of the system are controlled so that confronting electrodes are driven in the same phase to avoid have the confronting electrodes competing with the electrodes on the same side of the system.